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Investment Securities (Hedge Funds and Private Equity Funds) (Details) $ in Millions | Sep. 30, 2018 USD ($) fund | Dec. 31, 2017 USD ($)
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Hedge funds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Number of investments | 1 |
Private equity security |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Number of investments | 13 |
Hedge and private equity funds included in other long-term investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Largest single fund investment | $ | $ 59.6 | $ 54.9

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